1. Field of the Invention
This invention relates to a biological indicator, more specifically to a self-contained biological indicator for a hydrogen peroxide sterilizer or a hydrogen peroxide plasma sterilizer.
2. Description of the Related Art
Biological indicators (or "sterility indicators") are devices that are used to test the efficacy of sterilizers, such as those that are commonly used in hospitals for sterilizing medical instruments, glassware, etc. The indicators include a source of microorganisms, a culture medium, and a detector to indicate the presence or absence of viable microorganisms. The culture medium may also serve as the detector, with formation of a cloudy suspension indicating growth of microorganisms. In practice, the source of microorganisms, typically an absorbent paper strip that has been impregnated with a predetermined concentration of live microorganisms, is subjected to a sterilization process. Thereafter, the microorganism impregnated strip is placed in a sterile culture medium and incubated for a predetermined time at an appropriate temperature. At the end of the incubation period, the detector is used to determine whether any microorganisms survived the sterilization process. In some indicators, microorganism survival, which means the sterilization was incomplete, is shown by a change in color of the detector.
To simplify the sterilization test process and minimize the risk that external contamination would affect the test results, the elements of the biological indicator--microorganisms, culture medium, and detector--have sometimes been packaged in a way that permits the microorganism source, culture, and indicator to be combined without exposing the biological indicator to the non-sterile surroundings. A number of these so-called "self-contained biological indicators" (SCBI) have been described in the patent literature.
U.S. Pat. No. 3,440,144, issued Apr. 22, 1969, to H. W. Anderson discloses an SCBI that includes a heat-sealed bag which contains a sterilized culture medium in a closed ampule and source of bacterial spores on a piece of absorbent paper. The ampule has a spout that can easily be broken without removing the ampule from the bag. The bag is placed in a sterilizer along with items to be sterilized. After the sterilization is complete, the spout is broken off and the culture medium contacts the absorbent paper. The still-sealed bag is then placed in an incubator for a predetermined time, sufficient to permit any surviving spores to give a visual indication.
U.S. Pat. No. 3,661,717 issued May 9, 1972, to Robert L. Nelson, discloses an SCBI in which the culture medium is contained in an inner compartment that is in snug engagement with an outer compartment that also contains viable microorganisms and a detector that changes color in response to growth of the microorganism. The "snug engagement" minimizes voids in which sterilizing gases can be trapped. Release of the trapped gas after completion of the sterilization cycle can result in a false indication of sterility.
A number of later patents disclose refinements that purport to provide SCBI's that are easier to use, provide results more quickly, and/or give more accurate results (see, e.g., U.S. Pat. Nos. 4,416,984; 4,580,682; 4,717,661; 5,073,488; and 5,223,401).
These references all have in common the fact that they are adapted for use with traditional sterilizers, such as those that use steam, radiation, or ethylene oxide. A type of sterilizer that has a number of advantages over these traditional sterilizers is the hydrogen peroxide plasma sterilizer that is disclosed in U.S. Pat. No. 4,643,876, issued Feb. 17, 1987, to Paul T. Jacobs et al. This sterilizer combines the use of hydrogen peroxide and plasma to provide a sterilizer that avoids undesirable characteristics of earlier sterilizers that exposed devices to be sterilized, and, potentially, workers as well, to high temperatures, high levels of radiation, or toxic gases. Sterilizers that use hydrogen peroxide alone, without plasma, are also available. In this specification and the appended claims, we use "hydrogen peroxide sterilizers" to refer to both types of sterilizers.
A sterilizer test pack for use with hydrogen peroxide sterilizers is disclosed in European Patent Application 90310824.9, published Apr. 10, 1991. That reference does not disclose details of an SCBI that would be suitable for use with that type of sterilizer.
Hydrogen peroxide is commonly used to protect food from spoilage by inhibiting microorganism growth. In order to assess the effectiveness of hydrogen peroxide for that purpose, it may be important to measure the recovery of spores damaged by hydrogen peroxide. S. E. Wallen and H. W. Walker evaluated media and procedures for spore recovery (J. of Food Science 44,560 (1979)). They used a phosphate buffer containing catalase to decompose the hydrogen peroxide, after the spores were exposed to the hydrogen peroxide for a predetermined time. Hydrogen peroxide is also known to be a cleaning and disinfecting agent for hygienic articles, such as contact lenses. U.S. Pat. Nos. 4,585,488 and 4,748,992, issued to G. Giefer on Apr. 29, 1986 and June 7, 1988, respectively, disclose a system for cleaning and disinfecting contact lenses that includes disinfecting the lenses with a hydrogen peroxide disinfection solution and then decomposing residual hydrogen peroxide with an aqueous solution of a decomposition catalyst comprising dissolved catalase. Others have also addressed the problem of neutralizing hydrogen peroxide residues on contact lenses. Representative of these are U.S. Pat. No. 4,899,914; European Patent Application 86308559.3; Canadian Patent 1,297,403; and PCT Applications WO 86/07264; WO 91/12825; and WO 92/11041.